1. Field of the Invention
The present invention relates to an element for a ball valve provided in a fluid circuit for switching between a state in which a fluid can flow and a state in which a fluid is prevented from flowing, and to a method of manufacturing a ball valve element.
2. Description of the Prior Art
A ball valve element is a ball-shaped member that has a through-hole for passage of a hydraulic fluid. The element also has a recess formed on the outside surface of the element into which a stem fits for rotating the element. A ball valve element is usually produced by shearing off a piece of round bar, machining the sheared piece to the required ball shape, and drilling a hole through the valve element. Alternatively, the valve element can be cast or forged, then machined to shape and drilled.
In recent years, methods have been proposed for reducing the time it takes to manufacture ball valve elements, and for making the elements at a lower cost. In one such method disclosed by JP-B-6-21670, a press die is used to form pipe into a ball shape. This conventional method comprises a step in which a recess is formed for the valve stem, which is followed by a step in which the outer surface is formed into a spherical shape. However, in the latter step, the pipe has to be formed into the spherical shape with a spacer inserted into the recess. For this, the workpiece (the pipe) to be shaped has to be aligned precisely in the direction from which the spacer is inserted into the recess. This positioning of the workpiece is a time-consuming task that makes it difficult to raise operational efficiency. Another drawback is that the need to provide spacer material increases the structural complexity of the press die, and an error in positioning the workpiece can result in a defectively formed product or a damaged die.
Moreover, if in an effort to facilitate the positioning of the workpiece the spherical surface is formed without using a spacer, the result is a major difference in deformation resistance between the side where the recess has been formed and the opposite side where no recess has been formed. This can give rise to eccentrically formed recesses, the need to include a large machining tolerance since a precise spherical surface cannot be achieved, and other such problems.
An object of the present invention is to provide a ball valve element and a method of manufacturing a ball valve element that prevents the formation of eccentric recesses and eliminates the need to position the ball valve element workpiece material.